A coaxial resonator of the above type typically consists of a copper resonator rod and an aluminum housing therearound, one wall thereof being at a given space from the tip of the rod, whereby the capacitance between the rod tip and the wall forms a capacitative loading for the resonator. The other end of the rod has been short-circuited with the other, i.e. opposite conducting wall of the housing. The helix resonator differs from the coaxial resonator in principle only in that the inner conductor, i.e. the rod, has been wound in the form of a helical coil, in order to have smaller dimensions.
The coaxial and helical resonators are encumbered with a basic drawback, viz. of how to provide a sufficient thermal stability. In the operational environments, where great temperature variations may be expected, great center frequency drift might occur owing to changes in the structural dimensions due to thermal expansion, and there through, also in the electrical properties. Secondly, when the resonator is used in power applications, the resonator rod becomes strongly heated, particularly at the open end where the field strength is greatest. Said heating of the rod lengthens it and thus shortens the space between the tip of the rod and the wall of the housing. Typically, together with a temperature rise, the resonant frequency decreases; respectively, a drop in the temperature increases the resonant frequency.
In order to compensate for changes in the center frequency caused by temperature variation, a plurality of methods have been used. The methods are mainly based on the idea that since the oscillator circuit of the resonator consists of loading capacitance and inductance of the rod connected in parallel, the capacitance is adapted to be variable in the manner that it as completely as possible compensates for a change of the inductance. This is understandable because it is easier to affect capacitance than inductance. Therefore, the methods include endeavours to reduce loading capacitance according to temperature rise.
One of the most conventional ways is to arrange the distance between the end of the resonator rod and the top surface of the cover, to be appropriate, whereby, as the temperature changes, the spacing between the resonator rod and the top surface changes so that the resonant frequency remains as much unchanged as possible. In practice the spacing between the end of the resonator rod and the top surface of the cover has to be made very small, whereby a drawback is first that when said spacing is very small, the Q value of the resonator is decreased because the capacitance between the end of the rod and the top surface, i.e. the loading of the resonator grows. Moreover, if the spacing is made too small, this may result in a risk of a breakdown, in particular when the resonators are used in power applications, such as in transmitter filters of radio apparatus, because the maximum of the electric field of the resonator is, as is a well known fact, in the tip of the rod or of the helical coil. One more weakness found in this method is that the risk of breakdown increases when said space is reduced. A risk of breakdown and rapid deterioration of the Q value create an obstacle in aiming at complete compensation so that the compensation is under compensation in nature.
A second way known in the art is to place a bimetal strip on the tip of the rod resonator so that it is parallel to the top surface of the cover. As the temperature rises the strip bends off from the cover, thus reducing the loading capacitance according to the temperature. One of the drawbacks of said method is, just as in the first method, that the bimetal strip lowers the Q value of the resonator and that the bimetal is very difficult to work with. The bimetal strip may also be placed on the cover of the housing, though this is not a good place for it in that the temperature of the cover is much lower than the temperature of the tip of the compensator, whereby the bimetal will not conform to the temperature it should.
A third method is to select the materials so that the temperature changes very little affect the dimensions thereof. The selection concerns, above all, the material of the rod, for which is selected e.g. coated iron with a lower temperature coefficient than in the copper rod usually employed. In that case, a drawback is an increase of weight in a filter constructed from resonators.
European Patent Application No. 0,211,455 discloses a microwave cavity with a conical base plate (3) which is designed to move in responses to changes in ambient temperature such that the volume enclosed by the conical base varies in inverse proportion to temperature i.e. the higher the temperature the smaller the volume. This teaching is the opposite of that of the present invention in which the volume within the cover increases with increasing temperature.
International Patent Application No. 87/03745 discloses a microwave resonator having a cavity which comprises a temperature compensating member 26 the dimensions of which are such that it will increasingly bow into the cavity volume with increasing temperature which is the opposite teaching to that of the present invention.
U.S. Pat. No. 3,740,677 and 4,156,860 both disclose microwave cavities having movable temperature compensating discs similar to that disclosed in European Patent Application No. 0,11,455,
U.S. Pat. No. 3,873,949 discloses a cavity resonator having a hollow cupshaped compensation member secured in a wall of the cavity. However, this specification does not disclose the form of compensation plate or the means of attachment thereof to the cavity wall as disclosed in the present invention.